Hydrothermal formation of heavy rare earth element (HREE)–xenotime deposits at 100 °C in a sedimentary basin
Hydrothermal formation of heavy rare earth element (HREE)–xenotime deposits at 100 °C in a sedimentary basin
复制标题
沉积盆地 100 °C 下热液形成重稀土元素 (HREE)-磷钇矿床
作者：
L. Richter;L. Diamond;P. Atanasova;D. Banks;J. Gutzmer
多数稀土矿床形成于岩浆流体中，但在沉积盆地中也发现了重稀土元素（HREE）富集的热液xenotime矿床。由于xenotime是出了名的不溶性，这些不太为人所知的沉积物是在典型的盆地温度下形成的，还是由更热的岩浆热液流体流入形成的问题就产生了。西澳大利亚北部的布朗山脉地区拥有富含稀土元素和铀的xenotime矿床。矿体由断控热液石英- xentime角砾岩组成，与太古宙基底岩和上覆的古元古代砂岩相交。对流体包裹体的分析表明，由于减压沸腾，xenotime在非常低的温度下（在100至120℃之间）析出。包裹体中含有高浓度的钇（10−3 mol/kg）、稀土（1-7 × 10−5 mol/kg）和铀（4 × 10−5 mol/kg），在这些低温下与xenotime平衡，表明磷酸盐的可用性限制了xenotime的析出量。分析进一步确定so42 -和Cl -是促进稀土和铀溶解度升高的配体。这些发现表明，在盆地温度下，显著的稀土元素输运和沉积是可行的，从而提高了不整合背景下稀土元素勘探的潜力。此外，水相金属含量支持这种矿石流体与其他地方的世界级元古代不整合型铀矿床之间的成因联系。
Most rare earth element deposits form from magmatic fluids, but there have also been discoveries of heavy rare earth element (HREE)– enriched hydrothermal xenotime deposits within sedimentary basins. As xenotime is notoriously insoluble, the question arises as to whether these lesser-known deposits form at typical basin temperatures or by influx of much hotter magmatic-hydrothermal fluids. The Browns Range District in northern Western Australia hosts deposits of xenotime that are enriched in HREEs and also uranium. The ore bodies consist of fault-controlled hydrothermal quartz-xenotime breccias that crosscut Archean basement rocks and overlying Paleoproterozoic sandstones. Analyses of fluid inclusions show that the xenotime precipitated at remarkably low temperatures, between 100 and 120 °C, in response to decompression boiling. The inclusions contain high excess concentrations of yttrium (10−3 mol/kg), REEs (1–7 × 10−5 mol/kg), and uranium (4 × 10−5 mol/kg) in equilibrium with xenotime at these low temperatures, showing that availability of phosphate limited the amount of xenotime precipitated. The analyses further identify SO4 2– and Cl– as the ligands that facilitated the elevated REE and uranium solubilities. These findings establish that significant REE transport and deposition is feasible at basin temperatures, and hence they raise the potential of unconformity settings for REE exploration. Moreover, the aqueous metal contents support a genetic link between this type of ore fluid and world-class Proterozoic unconformity-related uranium deposits elsewhere.